Understanding the molecular mechanisms that regulate beta cell mass have important ramifications for fostering beta cell regeneration and the treatment of diabetes. Studies supported by this grant have established the importance of beta cell replication in regulating beta cell mass during growth, physiological expansion and regeneration. During the past funding cycle, we have shown that cyclin D2 plays a key role not only in establishing beta cell mass but also in adapting to insulin resistance. We established how metabolic changes lead to modulation of beta cell mass by mechanisms that regulate the cellular abundance of p27. Recently, we have shown that the age-dependent capacity of beta cells to replicate declines with age and is regulated by polycomb genes that control the levels of cell cycle regulator, p16Ink4a by epigenetic mechanisms. In the next five years we propose to design epigenetic strategies that can enhance beta cell replication in adults, test the requirements of beta cell replication in injury models of regeneration and develop new imaging tools to visualize beta cell replication during growth, development and regeneration. We propose to study: Aim 1, whether increased Ezh2 levels in beta cells in vivo can inhibit p16Ink4a, promote beta cell replication and enhance beta cell regeneration in adult mice; in Aim 2, study the mechanism by which Brg1 regulates beta cell replication in vivo by generating and analyzing mice that lack Brg1 in beta cells and carry out genome-wide analysis of Brg1 targets on cell cycle regulators in beta cells; in Aim 3, assess whether introduction of cyclin D2 into beta cells of cyclin D2 null mice is sufficient for beta cell growth and regeneration; in Aim 4, propose to monitor the spatiotemporal patterns of cell-cycle dynamics during pancreas development, growth in postnatal period and regeneration using a novel transgenic mouse system with fluorescent sensors. These studies will be carried out using null mouse mutants, beta cell-specific inducible transgenic mice, and cultured islet, using methods that are fully implemented in the laboratory. A major overall strength of this proposal is that we have already generated or obtained all the mice described here as well as assessed their breeding performance, and have carried out intercrosses to verify our ability to generate desired genotypes.